Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are deposited on opposite surfaces of an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode where it is ionized. The oxygen anions transport through the electrolyte and combine with hydrogen ions to form water. The cathode and the anode are connected externally through a load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the reformate gas includes CO which is converted to CO2 at the anode via an oxidation process similar to the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is usual to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously.
A typical SOFC assembly comprises a variety of metallic and non-metallic materials. Heated gases such as 02 and reformate are communicated around the assembly using metal tubes at internal pressures of, for example, 2.2 psig. The ends of these tubes are hard mounted as by welding, brazing, or bolting to a rigid member. As the tubes are subjected to extreme temperature excursions during start-up, operation, and shut-down of the assembly, significant dimensional changes in tube length can occur. Simultaneously, the SOFC assembly itself is subjected to high temperature excursions creating its own dimensional changes. Hard mounted tubing under these conditions is subjected to high stresses, both thermal and vibrational, which can cause buckling, bending, cracking, and failure of the tubular members. Catastrophic failure of these tubes allows the escape of explosive or reactive gases into the atmosphere.
It is a principal object of the present invention to prevent damage and failure in an SOFC assembly from thermal or vibrational overstress of tubing and piping.
It is a further object of the invention to reduce heat loss from the high temperature zone of the SOFC system.
It is a further object of the invention to increase the reliability and working lifetime of an SOFC assembly.